1. Field of the Invention
The present invention relate to a shift range switching device for a vehicle including an electric motor.
2. Description of the Related Art
Typically, in an automatic transmission vehicle, including a Continuously Variable Transmission (CVT) and a Dual Clutch Transmission (DCT), switching a shift range to P (parking), R (reverse), N (neutral) and D (drive) is performed by a driver's operation of a shift lever. The shift lever operates a manual shaft via a shift cable to operate a manual valve of the transmission, so that oil passages of respective ranges are switched. Hydraulic equipment for such as P, R, N D or the like are respectively operated by switching the oil passages, and the vehicle is thus operated.
Recently, a SBW (Shift-By-Wire) method has been proposed as a technology in which a manual shaft is directly operated by an electric motor without using the shift cable described above. However, a large torque is required to release a parking lock. For generating such a large torque, there are methods of increasing power density of an electric motor, employing a speed reducer, etc. However, these methods result in size increases of the devices and therefore, in some instances, it may not be possible to dispose these devices in a vehicle mounting space.
For that reason, a reduction mechanism has been proposed, in which a large reduction ratio can be obtained in a relatively small size, by using an internal gear and an external gear which is engaged with an outer circumference the internal gear and by rotating either the internal gear or the external gear by means of the electric motor.
However, in the aforementioned reduction mechanism, the number of required gears may become increased, or a mechanism that rotates either the internal gear or the external gear may become complicated, so that it is difficult to make the reduction mechanism compact and to reduce the number of components. In particular, in a case when another gear or the like, which is provided separately from an internal-toothed gear and an external-toothed gear, is used and an output after being reduced is drawn out to the outside, the number of required components is increased and configurations can be further complicated. For that reason, with the shift range switching device of a vehicle using this type of reduction mechanism, it is difficult to simplify and miniaturize the device.